Steam is commonly used in most process plants for providing heating/evaporation. The steam once used loses its latent heat and is converted into a condensate. The condensate thus formed, in a single application or multiple such applications at different locations, is pumped, typically at atmospheric pressure, to a feed water tank in a boiler house, a condensate recovery header or any other suitable equipment. The recovery of the condensate is essential to optimize the overall efficiency as well as to reduce the operational cost of the plant.
Saturated condensate at higher pressure flashes into steam typically known as flash steam when it is exposed to a lower pressure. The amount of flash steam generated increases with the increase in the differential pressure across the process traps. The reduced pressure of the condensate downstream of the process trap is insufficient to return the condensate, on its own, back to the feed water tank, and hence the requirement of a pump arises to pump this condensate back.
Most of the condensate flashes at the beginning of the downstream line of the process trap and increases with the pressure drop of the downstream line. If this condensate and flash steam is directly routed to the condensate pump (the pump here refers to a positive displacement pressure operated pump or a condensate recovery unit) the flash steam will get entrapped with the condensate and flow into the pump. In most practical cases the condensate loses its heat to the atmosphere as losses through the downstream line, and usually gets sub-cooled. This temperature difference causes the flash entrapped within the condensate to collapse leading to knocking or the phenomenon generally known as steam hammer.
Steam hammer within the pump causes vibrations due to high impact forces which can damage the internals of the pump, the return line to the feed water tank and can also cause structural damage to the pump shell and supports.
This problem inherently shows that the flash steam would have to be separated from the condensate before pumping. This separation of flash steam from the condensate is done by an appropriately sized vessel known as a flash vessel. The flash vessel separates the flash steam from the condensate, which can be used in any suitable application. The separated condensate then flows through a steam trap located at the condensate outlet (located typically at the bottom of the flash vessel), which ensures that flash steam cannot escape from the flash vessel through the condensate outlet to the pump receiver. The pump is usually located in a pit so that the condensate from the flash vessel trap can flow by gravity into the pump receiver or the flash vessel is raised to achieve the same.
The pressure at which the flash vessel is operated depends upon the applications in which the flash steam is utilized. However in most cases, wherever the flash is utilized in a suitable application, the flash vessel pressure is maintained above the atmospheric pressure. In applications where there are no suitable uses of flash steam or there is no practical feasibility of usage, the flash steam is vented to the atmosphere due to which the flash vessel is operated at atmospheric pressure.
The known systems for condensate and flash steam recovery exhibit several drawbacks. In these systems, if the steam trap fails to operate downstream of the flash vessel, in the closed condition, the flash vessel floods leading to steam hammer as the condensate level increases beyond the flash vessel inlet. This may also lead to a condition where the condensate backs up to the process, thereby affecting the associated process heating equipment, especially in situations where the condensate load is substantial.
When the steam trap fails in the open condition there is a possibility of flash steam escaping through the condensate outlet which is then vented to the atmosphere through the liquid dispenser. Also, the large number of joints and complex design of the existing condensate recovery systems increase the chances of leakage and thereby losses. It also leads to a larger footprint area and hence occupies more floor space.
Thus, there is felt a need for a system which reduces the above-listed drawbacks.